In a small town in Japan, dolphins are captured to be sold to SeaWorld type water shows. But the real crime is what happens to the dolphins who aren't chosen. They're taken to a secluded cove where no one can see them being slaughtered for food (food that is incredibly high in mercury, and basically toxic).
Richard O'barry is the star of the film. He made a fortune training the dolphins for "Flipper," became an activist after he witnessed the intelligence of the dolphins and was convinced they couldn't live happily in captivity. And the whole movie is a quest to get footage of what goes on in that cove.
And even if you're not motivated by the plight of dolphins in Japan, you can be amazed at the team they assemble and the spy tactics they use to get footage of the cove. Military grade night vision cameras (which are illegal to take out of America), cameras hidden in fake rocks (with the help of some ILM model makers), daring midnight missions to plant all of this, etc. And then the footage, as amazing and clear as it is shocking. There's an amazing scene from an underwater camera that starts out clear blue, a school of fish swim by. And then it slowly turns dark red from all the blood in the water. Nauseating, and amazing.